Many industries use precisely shaped parts. For example, conventional blades for use in gas turbine engines must be precisely shaped. Typically, precision investment casting or die-forging forms a metal starting blank into a blank having a blade shape.
Generally, the blank includes a tip, a root region longitudinally spaced therefrom, and a midspan region extending between the tip and the root region. A platform typically separates the root region from the midspan region. The tip may include an attached or integral shroud. A number of details must be machined into these portions of the blank, to put the precisely shaped part is in its final configuration. For example, a plurality of dove tails must be machined into the root region. These dove tails allow the blade to be secured to the rotor disk in the engine. In order to achieve acceptable engine performance, these dove tails must be formed within small tolerances. To machine the dove tails into the blank with the necessary accuracy, there must be references from which to measure the configuration of the dove tails. The references used, dictate the type of fixture that will be used to hold the blank during machining.
Various methods for providing such references and holding the blank have been suggested. The references may be provided as taught in U.S. Pat. No. 2,577,747 issued to Gibian. Gibian teaches forging at least two hemispherical buttons or protrusions into the blank. Both buttons are disposed on the blank along the stacking axis. Using these buttons, the root region can be ground down to a predetermined thickness, and a bore can be drilled into the opposite end of the blank. Using a lathe as the fixture, the blank is held by clamping the root region and using a pin to engage the bore. The leading and trailing edges of the blank are machined using the thickness of the root region and the bore as references.
The most significant problem with the solution taught in Gibian, is that machining the bore and the thickness of root region based on the buttons does not provide sufficiently accurate results. Another problem is that the root region of the blank cannot be machined while being held in the lathe.
References and fixturing may also be provided as taught in U.S. Pat. No. 3,818,646 issued to Peterson. Peterson teaches a locating button on the root portion of the blade disposed along a design axis of the blank, such as the stacking axis. A fixture utilizes this button as a reference. While in the fixture, datum planes may be ground or machined into the root and shroud portions of the blank to define accurately located surfaces for subsequent machining.
One problem with this solution is that the fixture employs clamping mechanisms that clamp the blank along the airfoil-shaped midspan. As a result, the fixtures are complex and therefore expensive to design, manufacture and maintain. Furthermore, clamping along the midspan makes the fixture dependent on the size and shape of the blade. Due to this dependency a number of fixtures are necessary to make all the blades in one engine, since an engine has several different size and shaped blades. Another problem with using a blade dependent fixture is that during production, time may be wasted changing between fixtures, thus significantly limiting the number of blades that can be manufactured in a period of time.
Another possible way of providing references is by encapsulating the blank in a block of material, such as a low melt alloy. The block of material is formed around the blank, so that the root region extends from the block. The sides of the block provide reference planes from which the configuration of the dove tails or other details can be determined with the necessary accuracy. However, encapsulating the blank requires a complex encapsulation tool. In addition, a complex fixture is necessary to hold the block containing the blank during machining of the blank. Different encapsulation tools and holding fixtures are required for each differently shaped or sized blank. Furthermore, after machining the dove tails into the blank the material must be melted off the blank and must be disposed of without causing environmental problems.
Therefore, an improved method is sought for forming a precisely shaped part. The method should make use of a blank that provides references to accurately machine details into the blank. A fixture for use with the blank must be inexpensive to design and maintain, and easily modifiable to accommodate different size or shape blades.